feat(proxy): enhance Docker security and add health check

[Ahoo-Wang]

• Add non-root user (appuser) and group (appgroup) for container security
• Copy application files with proper ownership (appuser:appgroup)
• Implement HEALTHCHECK using curl to monitor actuator endpoint
• Set STOPSIGNAL to SIGTERM for graceful shutdown
• Switch from root to appuser for running the application

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.27%. Comparing base (fa215c0) to head (ab88025).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1002      +/-   ##
============================================
- Coverage     85.32%   85.27%   -0.05%     
  Complexity     1328     1328              
============================================
  Files           214      214              
  Lines          4191     4191              
  Branches        413      413              
============================================
- Hits           3576     3574       -2     
- Misses          341      342       +1     
- Partials        274      275       +1     

Flag	Coverage Δ
unittests	85.27% <ø> (-0.05%)	⬇️

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